Method of producing grain-oriented



United States Patent 3,125,473 METHDD 0F PRODUCING GRAIN-ORIENTED SILICON STEEL Henry C. Schneider, Penn Hills Township, Allegheny County, Harry F. Shannon, Monroeville Borough, and Edward B. Stanley, Washington Township, Westmoreland County, Pa, assignors to United States Steel Corporation, a corporation of New Jersey No Drawing. Filed Mar. 31, 1961, Ser. No. 99,665

Claims. (Cl. 148-111) This invention relates to an improved method of producing grain-oriented silicon steel and more particularly to a method of producing grain-oriented silicon steel having improved punching characteristics.

Grain-oriented silicon steel for use in the cores of electrical equipment, such as power transformers, turbinegenerators, and similar applications, is die-cut or punched into laminations of the required shape which are then assembled to form the magnetic core of the equipment. During the processing of grain-oriented steel by conventional methods, however, a glass-like film is formed on the surface of the steel. This film is very abrasive; consequently the life of the dies used in punching laminations of grain-oriented steel is very short.

To improve the punching properties of grain-oriented steel, producers have resorted to a final pickling treatment before shipment to remove the abrasive surface film. Because of its glass-like nature, however, the film is highly resistant to attack by acids. We have found that by use of a particular atmosphere in the second continuous annealing of grain-oriented steel, a surface film is produced which can be easily removed by pickling.

The conventional procedure for producing grainoriented steel is as follows:

(1) Hot roll steel containing between 3.0 and 3.5% siiicon to about .08" gauge to produce a hot-rolled band (2) Pickle hot-rolled band in a strong acid solution to remove the hot mill scale (3) Cold reduce the hot-rolled band to about .024 to .028" gauge to produce cold-rolled strip (4) Continuously anneal the cold-rolled strip at temperature between about 1450 and 1700 F. in a reducing atmosphere (5) Cold reduce the strip to .012 to .014" gauge (6) Continuously anneal the double cold reduced strip at a temperature between about 1400 to 1600 F. in an atmosphere containing essentially H and water vapor in at least a 3 to 1 ratio (7) Apply a finely ground magnesium oxide separating medium coating in an amount of about 0.002 to 0.015 02/ sq. ft. of surface area (8) Box anneal at a high temperature between about 1800 to 2200 F., preferably about 2050 F., in hydrogen final gauge, e.g., to about The present invention is primarily concerned with the second continuous anneal, i.e. step No. 6, in the above recited sequence of steps. The object of the second continuous anneal is to recrystallize the cold-worked grains and to remove as much carbon from the steel as possible. To achieve the required decarburization, an atmosphere containing hydrogen and water vapor as active ingredients is conventionally used. The quantities of these two gases present in the furnace atmosphere are adjusted to permit decarburization of the steel without excessive oxidation of the iron. The ratio of hydrogen to water used for this purpose is at least 2 to 1 or higher, preferably about 3 to 1. With higher ratios less iron is oxidized but less carbon is removed.

With such mixtures of hydrogen and water vapor, sili- 3,125,473 Patented Mar. 17, 1964 "ice con from the steel forms a practically continuous layer of silica on the surface of the steel. During the subsequent high-temperature anneal in hydrogen, this layer of silica combines with the magnesium oxide to form a glasslike film of magnesium silicate. The small amount of iron oxide formed during the continuous anneal is reduced to iron in the high temperature anneal and may combine with the magnesium silicate to form a complex iron-magnesium silicate. The glass-like film, whether magnesium silicate or iron-magnesium silicate, is very abrasive and is highly resistant to acids. The film is deliberately produced for applications requiring high surface insulation, but it has a very detrimental efliect on punching dies, as mentioned previously, and, therfeore, must be removed before the steel is punched into laminations. The high temperature box anneal also develops undesirable coil set which must be removed before the material can be used for laminations.

Heretofore, this glass-like film has been removed by pickling the steel in acid, which, because of the long times involved, was very costly. We have discovered that by changing the atmosphere of the second continuous anneal to one containing essentially nitrogen, oxygen and water vapor, the film on the fully processed steel can be readily and completely removed in a very short time, i.e. less than about 20 minutes by pickling in an acid solution, such as a 50% solution of hydrochloric acid. Sutficient decarburization occurs during continuous annealing in our improved atmosphere so that the final product has the desired magnetic properties.

The surface film formed in the atmosphere of our invention is chiefly iron oxide and some silica, formed by the oxidation of silicon from the steel. As mentioned previously, the film formed in the conventional atmosphere is chiefly silica with a minor amount of iron oxide. Thus, the film formed on the final product of the new practice is less silicious and is more readily attacked by acids than is the film on the product of the present commercial practice.

Although we have mentioned nitrogen specifically as the major component of the atmosphere, the new atmosphere may, also, contain argon, helium, or other so-called neutral gases in combination with or instead of nitrogen. When nitrogen is used, the moisture content should be about 1% or higher to clecarburize the steel to the desired low level, about 0.004% maximum carbon.

With nitrogen as the base gas, the broadest range over which the moisture content can vary is 1 to 3%, the most likely commercial range is 1 to 2%, and the optimum amount is 1 /2% (dew point of +55 F.). A small amount of hydrogen may be present as an impurity but if present, the ratio of hydrogen to water vapor must not exceed 2 to 1. Oxygen should be present in effective amounts up to 0.5% and preferably in the range of .2 to .4%.

Thus our process is as follows:

(1) Hot roll steel containing between 3.0 and 3.5% silicon to about .08" gauge (2) Pickle hot-rolled band in a strong acid solution to remove the hot mill scale (3) Cold reduce to about .024 to .028" gauge (4) Continuously anneal at temperature between about 1450 and 1700 F. in a reducing atmosphere (5) Cold reduce to final gauge, e.g., to about .012 to .014" gauge (6a) Continuously anneal at 1400 to 1600 F. in an atmosphere of essentially NH O with moisture present in the range of 1 to 3%. If a small amount of hydrogen is present, the ratio of H to H O must not exceed 2 to 1 (7) Apply SM coating (MgO solution or slurry of sufficient concentration to produce a final coating of 0.002 to 0.015 oz./sq. ft. of strip surface area) (8) High-temperature box anneal at a temperature between 1800 and 2200 F. and preferably at about 2050 F. for 10 to 48 hours (9) Third continuous anneal at 900 to 1450 F. to remove coil set (10) Pickle to remove SM coating. HCI, HNO H SO or mixtures thereof can be used. Bath contains 10 to 75% acid by weight, lower concentrations being preferable for economic reasons. Bath temperatures of 32 to 200 F. may be used, preferably room temperature. At such temperature, 20 to 30 minutes pickling time will suffice. If desired the bath may be heated to shorten the time.

To illustrate the teachings of our invention, two samples A and B were processed through steps 1 to 5. Sample B was then treated according to our invention by continuous annealing in an atmosphere containing specifically, 1.9% moisture and remainder commercially pure nitrogen (step (6a)) containing about .3% oxygen. The other sample A was annealed according to step 6. Both samples were then treated according to steps 7 and 8. Step 8 was 7 /2 hours at 2150 F. (laboratory conditions approximating the commercial conditions of step 8 above). Both samples were pickled according to step 1050% HCl for 20 minutes. Sample A remained 40% covered with scale. Sample B was completely stripped of scale.

Experiments have also been conducted with atmospheres containing (a) 12% carbon dioxide, remainder nitrogen with a +70 F. dew point and (b) high purity nitrogen having a dew point of about ;+70 F. In these experiments samples of silicon steel containing about 325% silicon having a gauge of .014" were processed in accordance with steps 1 to 5. The steel was then continuously annealed at 1475 F. for about minutes following which it was coated with .02 to .025 oz./sq. ft. of magnesium oxide and box annealed at about 2150 F. for 7 /2 hours in a hydrogen atmosphere having a 60 F. dew point. The samples continuously annealed in atmosphere (a) showed only slight removal of oxide after pickling for about minutes in a 50% hydrochloric acid solution at 120 F. and those annealed in atmosphere (b) similarly pickled showed only traces of oxide removed after 15 minutes.

Punching grade grain-oriented silicon steel should be such that a die can punch out a minimum of about 50,000 laminations before developing an objectionable burr of about 0.005 inch. Tests show that the product of the conventional process causes an objectionable burr on the die after 3000 to 10,000 punches. The product of our process B does not cause an objectionable burr on the dies until after about 50,000 to 100,000 laminations have been punched.

This application is a continuation-in-part of our copending application Serial No. 750,090, filed July 22, 1958, now abandoned.

While we have shown and described a specific embodiment of our invention, it will be understood that this embodiment is merely for the purpose of illustration and description and that various other forms may be devised within the scope of our invention, as defined in the appended claims.

We claim:

1. A method of producing grain-oriented silicon steel characterized by a nonabrasive surface film comprising cold-reducing hot-rolled silicon electrical steel in two stages, annealing the steel intermediate said cold reductions in a reducing atmosphere and after the second cold reduction continuously annealing the steel in an oxidizing atmosphere consisting essentially of nitrogen, effective amounts up to .5 oxygen and 1 to 3% water vapor, coating the continuously annealed steel with a magnesium oxide separating medium, box annealing it in a reducing atmosphere and pickling in an aqueous acid solution for less than about 30 minutes to strip the scale therefrom.

2. In a method of producing grain-oriented silicon steel characterized by a nonabrasive surface film including cold-reducing hot-rolled silicon electrical steel in two stages, annealing the steel intermediate said cold reductions in a reducing atmosphere the improvement comprising continuously annealing the steel in an atmosphere consisting essentially of a neutral gas, effective amounts up to .5% oxygen and l to 3% water vapor after the second cold reduction, coating the continuously annealed steel with a magnesium oxide separating medium, box annealing it in a reducing atmosphere, said continuous anneal in the oxidizing atmosphere and said box anneal with the magnesium separating medium combining to produce a scale on said strip which is readily removable by an acid pickling treatment.

3. In a method of producing grain-oriented silicon steel characterized by a nonabrasive surface film including cold-reducing hot-rolled silicon electrical steel in two stages, annealing the steel intermediate said cold reductions in a reducing atmosphere the improvement comprising continuously annealing the steel in an atmosphere consisting essentially of nitrogen, .2 to .4% oxygen and 1 to 3% water vapor, coating the continuously annealed steel with a magnesium oxide separating medium, box annealing it in a reducing atmosphere and thereafter pickling it in an aqueous acid solution for less than about 30 minutes whereby the scale is completely removed therefrom.

4. A method of producing grain-oriented silicon steel characterized by a nonabrasive surface film comprising cold-reducing hot-rolled electrical steel containing between 3.0 and 3.5% silicon in two stages, annealing the steel intermediate said cold reductions in a reducing atmosphere and after the second cold reduction continuously annealing the steel in an oxidizing atmosphere consisting of effective amounts up to .5 oxygen, 1 to 3% water vapor and the balance nitrogen, coating the continuously annealed steel with a magnesium oxide separating medium, box annealing it in a reducing atmosphere and pickling in an aqueous acid solution for less than about 30 minutes to strip the scale therefrom.

5. In a method of producing grain-oriented silicon steel characterized by a nonabrasive surface film including cold-reducing hot-rolled electrical steel containing between 3.0 and 3.5% silicon in two stages, annealing the steel intermediate said cold reductions in a reducing atmosphere the improvement comprising continuously annealing the steel in an atmosphere consisting of .2 to .4% oxygen, 1 to 3% Water vapor and the balance nitrogen, coating the continuously annealed steel with a magnesium oxide separating medium, box annealing it in a reducing atmosphere and thereafter pickling it in an aqueous acid solution for less than about 30 minutes whereby the scale is completely removed therefrom.

References Cited in the file of this patent UNITED STATES PATENTS 2,304,518 Williams Dec. 8, 1942 2,455,632 Williams Dec. 7, 1948 2,535,420 Jackson Dec. 26, 1950 2,656,285 Burns et a1 Oct. 20, 1953 2,906,645 Carpenter et al Sept. 29, 1959 

1. A METHOD OF PRODUCING GRAIN-ORIENTED SILICON STEEL CHARACTERIZED BY A NONABRASIVE SURFACE FILM COMPRISING COLD-REDUCING HOT-ROLLED SILICON ELECTRICAL STEEL IN TWO STAGES, ANNEALING THE STEEL INTERMEDIATE SAID COLD REDUCTIONS IN A REDUCING ATMOSPHERE AND AFTER THE SECOND COLD REDUCTION CONTINUOUSLY ANNEALING THE STEEL IN AN OXIDIZING ATMOSPHERE CONSISTING ESSENTIALLY OF NITROGEN, EFFECTIVE AMOUNTS UP TO .5% OXYGEN AND 1 TO 3% WATER VAPOR, COATING THE CONTINUOUSLY ANNEALED STEEL WITH A MAGNESIUM OXIDE SEPARATING MEDIUM, BOX ANNEALING IT IN A REDUCING ATMOSPHERE AND PICKLING IN AN AQUEOUS ACID SOLUTION FOR LESS THAN ABOUT 30 MINUTES TO STRIP THE SCALE THEREFROM. 